The present invention relates to color picture tubes having inline electron guns, and particularly to an improvement in such guns having expanded lenses for obtaining increased static convergence.
An inline electron gun is one designed to generate or initiate preferably three electron beams in a common plane and direct those beams along convergent paths to a point or small area of convergence near the tube screen. In one type of inline electron gun, such as that shown in U.S. Pat. No. 3,873,879, issued to Hughes on Mar. 25, 1975, the main electrostatic focusing lenses for focusing the electron beams are formed between two electrodes referred to as the first and second accelerating and focusing electrodes. These electrodes include two cup-shaped members having the bottoms of the members facing each other. Three apertures are included in each cup bottom to permit passage of three electron beams and to form three separate main focus lenses, one for each electron beam. In a preferred embodiment, the overall diameter of the electron gun is such that the gun will fit into a 29 mm tube neck. Because of this size requirement, the three focusing lenses are very closely spaced relative to each other thereby providing a severe limitation on focus lens design.
An improved electron gun having an expanded focus lens to overcome the aforementioned focus lens design limitation is disclosed in copending U.S. patent application Ser. No. 201,691, filed Oct. 29, 1980 by Hughes et al. which is assigned to the same assignee as the present application. In this electron gun, the main focus lens is formed by two spaced electrodes. Each electrode includes a plurality of apertures therein equal to the number of electron beams. Each electrode also includes a peripheral rim with the peripheral rims of the two electrodes facing each other. The apertured portion of each electrode is located within a recess set back from the rim. Each aperture of each focus electrode includes a peripheral extrusion which extends into the electrode away from the opposing electrode. Static convergence of the two outer electron beams with the center electron beam is accomplished by forming a wider peripheral rim on one focus electrode than on the other focus electrode.
It has been found that the foregoing static convergence means only provides about forty percent of the static convergence required. Therefore, there is a need for a static convergence scheme that will provide greater static convergence. Furthermore, the use of differently shaped parts for electron gun construction, especially where the parts are not easily distinguishable by visual inspection, is undesirable both because of additional cost and because of the possibility of assembly errors.